Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for an image pickup optical system used for an image pickup apparatus such as a digital camera, a video camera, a TV camera, a monitoring camera, or a silver-halide film camera.
Description of the Related Art
As an image pickup optical system used for an image pickup apparatus (camera), a zoom lens having a wide angle of view, high resolution power, and a small size is required. The zoom lens is also required to be able to execute autofocusing (automatic focusing) at high speed and with high accuracy. As an autofocus system for use in picking up a still image, a phase difference system is often used. Meanwhile, a single-lens reflex camera in recent years is required to have a moving image capturing function and be able to perform autofocusing while capturing a moving image.
As an autofocus system for use in capturing the moving image, a system in which drive sound generated in driving a focus lens unit is small and which is capable of executing focusing at high speed is required. As an autofocus system that satisfies those requirements, a high-frequency detection system (TV-AF system) in which a high-frequency component in an image pickup signal is detected to evaluate an in-focus state of an image pickup optical system has been known.
In an image pickup apparatus using the TV-AF system, the focus lens unit is oscillated in an optical axis direction at high speed (hereinafter referred to as “wobbling”) to detect a direction of a shift from the in-focus state. Then, after the wobbling, a signal component in a particular frequency band in an image region is detected from an output signal of an image sensor to calculate an optimal position of the focus lens unit in the optical axis direction at which the in-focus state is obtained. Thereafter, the focus lens unit is moved to the optimal position to complete focusing.
In capturing the moving image, the focus lens unit needs to be driven at high speed and as quietly as possible so that drive sound of a motor, in particular, is not recorded. Therefore, the focus lens unit is strongly required to be small in size and weight. Heretofore, as a zoom lens having a wide angle of view and a small size as an entire system, there has been known a negative lead type zoom lens in which a lens unit having a negative refractive power is arranged closest to an object side. As the negative lead type zoom lens, there has been known a zoom lens in which a lens unit that is small in size and weight is used to perform focusing.
In Japanese Patent Application Laid-Open No. 2012-173298, there is disclosed a zoom lens including, in order from an object side to an image side, a first lens unit to a fourth lens unit having negative, positive, negative, and positive refractive powers, in which an interval between each pair of adjacent lens units is changed to perform zooming, and in which the third lens unit is used to perform focusing. In Japanese Patent Application Laid-Open No. 2012-27283, there is described a zoom lens including, in order from an object side to an image side, a first lens unit to a fifth lens unit having negative, positive, negative, negative, and positive refractive powers, in which an interval between each pair of adjacent lens units is changed to perform zooming, and in which the first lens unit or the fourth lens unit is used to perform focusing.
The zoom lens used for the image pickup apparatus is strongly required to have the wide angle of view, have a small size as the entire system of the zoom lens, include the focus lens unit being small in size and weight, be capable of performing focusing at high speed, be small in aberration variations during focusing, and the like. In order for the focus lens unit to be small in size and weight, it is required to reduce the number of constituent lenses of the focus lens unit. However, when the number of constituent lenses of the focus lens unit is reduced, residual aberrations in the focus lens unit are increased. Therefore, the aberration variations during focusing are increased, and hence it becomes difficult to obtain good optical performance over the entire object distance from a long distance to a short distance.
Meanwhile, when a power of the focus lens unit is reduced in order to reduce the aberration variations during focusing, a movement amount during focusing is increased, and a total length of the zoom lens is increased. In order to obtain the zoom lens having the small size as the entire system, having the wide angle of view, being capable of performing focusing at high speed, and having small aberration variations during focusing, it is important to appropriately set the number of lens units, the refractive powers of the respective lens units, and a selection, a lens configuration, and the like of the focus lens unit.
In the negative lead type zoom lens, in particular, it is important which of a plurality of lens units constituting the zoom lens is selected as the focus lens unit. It is also important to appropriately set a refractive power of the focus lens unit, the movement amount of the focus lens unit during zooming, and the like. When those elements are not appropriately set, it becomes difficult to obtain the zoom lens having the small size as the entire system, having the wide angle of view, and having high optical performance over the entire object distance.